build: add GN configurations for simdjson

deps/simdjson/BUILD.gn

@@ -0,0 +1,14 @@
+##############################################################################
+#                                                                            #
+#                       DO NOT EDIT THIS FILE!                               #
+#                                                                            #
+##############################################################################
+
+# This file is used by GN for building, which is NOT the build system used for
+# building official binaries.
+# Please modify the gyp files if you are making changes to build system.
+
+import("unofficial.gni")
+
+simdjson_gn_build("simdjson") {
+}

deps/simdjson/unofficial.gni

@@ -0,0 +1,22 @@
+# This file is used by GN for building, which is NOT the build system used for
+# building official binaries.
+# Please edit the gyp files if you are making changes to build system.
+
+# The actual configurations are put inside a template in unofficial.gni to
+# prevent accidental edits from contributors.
+template("simdjson_gn_build") {
+  config("simdjson_config") {
+    include_dirs = [ "." ]
+  }
+
+  gypi_values = exec_script("../../tools/gypi_to_gn.py",
+                            [ rebase_path("simdjson.gyp") ],
+                            "scope",
+                            [ "simdjson.gyp" ])
+
+  source_set(target_name) {
+    forward_variables_from(invoker, "*")
+    public_configs = [ ":simdjson_config" ]
+    sources = gypi_values.simdjson_sources
+  }
+}

unofficial.gni

@@ -159,6 +159,7 @@ template("node_gn_build") {
       "deps/nghttp2",
       "deps/ngtcp2",
       "deps/postject",
+      "deps/simdjson",
       "deps/simdutf",
       "deps/uvwasi",
       "//third_party/zlib",